1. Field of the Invention
The present invention relates to a photovoltaic device manufacturing method. More specifically, the invention relates to a method and apparatus for manufacturing a photovoltaic device with a collector electrode formed from a thin metal wire on a photovoltaic member surface.
2. Related Background Art
The electrode structure of a photovoltaic device is mainly made up of a comb-like or lattice-like collector electrode, which is relatively thin and formed from metal to collect current, and a bus bar, which is a relatively thick electrode formed from metal to collect the current collected by the collector electrode. Preferable materials of the electrodes are ones that are low specific resistance, for example, silver and copper.
Methods preferred in forming these electrodes, in particular, the collector electrode, include evaporation, plating, and screen printing.
However, forming the electrodes by evaporation presents such problems as slow deposition rate, low throughput resulting from a vacuum process, and a waste of metal deposited on a mask, which is necessary to form a linear pattern.
Screen printing also has a problem in that high specific resistance of electroconductive paste used in printing, 4.0×10−5 Ωcm at the lowest, makes it difficult to obtain an electrode that is low in specific resistance. Generally speaking, electroconductive paste has a specific resistance one-digit higher than that of pure silver bulk.
Furthermore, evaporation and screen printing need to make a collector electrode thicker in order to lower the resistance of the electrode without changing the area of the electrode, for example. The practical thickness of a collector electrode is 1 μm or less in evaporation and 10 μm to 20 μm in screen printing, and a collector electrode of such thickness measures 200 μm or more in width, resulting in very small aspect ratio (length-breadth ratio) and large shadow loss.
Those are contrasted by a collector electrode disclosed in Japanese Patent Application Laid-Open No. H07-335921 and Japanese Patent Application Laid-Open No. H11-145502. This collector electrode has a thin metal wire covered with resin that contains electroconductive particles. According to the publications, a thin wire of highly conductive metal such as copper is used to form the collector electrode. Therefore, the electric resistance loss remains small when the collector electrode is long and, in addition, the aspect ratio can be set to 1:1 to make shadow loss small. Moreover, the thin wire is fixed by a simple method which uses electroconductive resin.
FIGS. 6A to 6E are schematic diagrams showing a method of forming a collector electrode of a photovoltaic device which is disclosed in Japanese Patent Application Laid-Open No. H11-145502. Referring to the drawings, the related art example is described below.
(a) A piece of double-sided adhesive tape 606 is placed on each end of the principal surface of a photovoltaic member 601, which is composed of a substrate and a photovoltaic layer formed on the substrate (FIG. 6A).
(b) The entire photovoltaic member 601 is bent and, in this state, thin metal wires 602 coated with electroconductive resin are placed in lines on the photovoltaic member 601 while fixing each end of the respective thin metal wires 602 to the double sided tape 606. A bus bar electrode 607 formed from a copper film is placed on the wires 602 fixed to the tape 606 (FIG. 6B). FIG. 6F is a sectional view taken along the line 6F—6F in FIG. 6B.
(c) The photovoltaic member 603 is put on a heat plate 603, which is kept heated (FIG. 6C).
(d) The photovoltaic member 601 and the heat plate 603 are covered with a chamber 605 whose bottom face is constituted of an elastic film 604. At this point, the air in the chamber 605 is exhausted by exhaust means (not shown) from an exhaust vent (not shown) in order to prevent the elastic film 604 from being stretched downward by its own weight and brought into contact with the thin metal wires 602, which are strained like a bowstring, thereby sagging or breaking the thin metal wires 602. The exhaust means creates a difference in pressure between the air below the elastic film 604 and the air above the elastic film 604 (inside an upper portion of the chamber) to pull the elastic film 604 upward (FIG. 6D).
(e) The air below the elastic film 604 is exhausted from outlets 608 opened in the heat plate 603 while air is let in the chamber 605 to bond the thin metal wires 602 by thermocompression to the surface of the photovoltaic member 601.
According to the methods of Japanese Patent Application Laid-Open No. H07-335921 and Japanese Patent Application Laid-Open No. H11-145502, the photovoltaic member 601 and the thin metal wires 602 are heated by pressing the photovoltaic member 601 against the heat plate 608, which is heated and kept at a given temperature. Heating of the photovoltaic member 601 is stopped by moving the photovoltaic member 601 away from the heat plate 608. The photovoltaic member 601 thus can be heated and cooled in a short period of time and therefore the methods are suitable for mass production.
The method according to Japanese Patent Application Laid-Open No. H07-335921 has a problem in that the thin metal wires 602 are bent when bonded by thermocompression to the photovoltaic member 601. To avoid bending the thin metal wires 602, the method according to Japanese Patent Application Laid-Open No. H11-145502 applies a tensile force to the thin metal wires 602 prior to thermocompression bonding of the thin metal wires 602 to the photovoltaic member 601.
In this method, however, the thin metal wires 602 could be bent and bonded to the top face of the photovoltaic member 601 if the tensile force applied to the thin metal wires 602 is lessened before the thin metal wires 602 are bonded and fixed to the top face of the photovoltaic member 601.
If the thin metal wires 602 are bent and bonded to the top face of the photovoltaic member 601, a current collection path is lengthened and accordingly Joule loss is increased. Furthermore, if the thin metal wires 602 are bent and bonded to a light incidence face of the photovoltaic member 601, shadows of the thin metal wires 602 projected onto the light incidence face takes up a large area and reduces the amount of electricity generated by the photovoltaic device.
Therefore, the first improvement that is desired to be made on the related art is to avoid bending the thin metal wires 602 so that an increase in Joule loss and lowering of electricity generated by the photovoltaic device are prevented.
Japanese Patent Application Laid-Open No. 2003-039554 also discloses a collector electrode having a thin metal wire covered with resin that contains electroconductive particles. According to the publication, a thin wire of highly conductive metal such as copper is used to form the collector electrode. Therefore, the electric resistance loss remains small when the collector electrode is long and, in addition, the aspect ratio can be set to 1:1 to make shadow loss small. Moreover, the thin wire is fixed by a simple method which uses electroconductive resin.
FIGS. 14A and 14B, FIGS. 15A to 15C, FIGS. 16AX to 16DX, and FIGS. 16AY to 16DY are schematic diagrams showing a conventional method of forming a collector electrode that uses a thin metal wire. Referring to the drawings, the related art example is described below.
In FIGS. 14A and 14B, reference numeral 1501 denotes a photovoltaic member which generates electromotive force upon incidence of light. FIG. 14A is a plan view of the photovoltaic member 1501 and FIG. 14B is a side view of the photovoltaic member 1501. A piece of double-sided adhesive tape 1507 is placed on each end of the principal surface of the photovoltaic member 1501. Thin metal wires 1502 coated with electroconductive resin are placed in lines on the photovoltaic member 1501 while fixing each end of the respective thin metal wires 1502 to the double sided tape 1507. In fixing the ends of the thin metal wires 1502 to the double-sided tape 1507, an appropriate tensile force is applied to the thin metal wires 1502 to make the thin metal wires 1502 taut. A bus bar electrode 1508 formed from a copper film is placed on the wires 1502 fixed to the tape 1507, thereby completing a complex 1503.
In Japanese Patent Application Laid-Open No. 2003-039554, the complex 1503 is put in a thermocompression bonding device to bond the thin metal wires 1502 by thermocompression to the photovoltaic member 1501.
FIGS. 15A to 15C are diagrams showing the thermocompression bonding device of Japanese Patent Application Laid-Open No. 2003-039554. The thermocompression bonding device is composed of heat plate 1604, a chamber 1609, and an elastic sheet 1606, which is attached to the chamber 1609. FIG. 15A is a sectional view of the thermocompression bonding device taken along the line 15A—15A of FIG. 15B and viewed from above, and corresponds to a plan view of the heat plate 1604. FIG. 15B is a sectional view of the thermocompression bonding device taken along the line 15B—15B of FIG. 15A. FIG. 15C is a sectional view of the thermocompression bonding device taken along the line 15C—15C of FIG. 15A. The chamber 1609 is moved up and down and is placed on top of the heat plate 1604. The heat plate 1604 has an elastic sheet suction groove 1612. When the chamber 1609 is placed on top of the heat plate 1604, an O-ring 1619 is fit in the elastic sheet suction groove 1612 to create an airtight space between the heat plate 1604 and the elastic sheet 1606. The heat plate 1604 also has deaeration holes 1613, which are provided to remove the air from the airtight space created between the heat plate 1604 and the elastic sheet 1606. The elastic sheet suction groove 1612 and the deaeration holes 1613 are connected to a vacuum pump 1618 through pipes 1614 and 1615 and through valves 1616 and 1617, respectively. As shown in an enlarged view in FIG. 15C, holes 1610 are opened in the heat plate 1604, allowing movable support members 1605 to protrude from the principal surface of the heat plate 1604. The movable support members 1605 are kept protruded unless an external force is applied by springs 1611.
FIGS. 16AX to 16DX and FIGS. 16AY to 16DY show a process of processing the complex 1503 in the thermocompression bonding device of FIGS. 15A to 15C. The process proceeds from FIG. 16AX to FIG. 16DX and from FIG. 16AY to FIG. 16DY. FIGS. 16AX to 16DX show sectional views of the thermocompression bonding device taken along the line 15B—15B of FIG. 15A. FIGS. 16AY to 16DY show sectional views of the thermocompression bonding device taken along the line 15C—15C of FIG. 15A.
In FIGS. 16AX and 16AY, an automatic hand (not shown) puts a complex 1703 on movable support members 1705 protruding from the principal surface of a heat plate 1704. The heat plate 1704 is kept heated by means that is not shown in the drawings. At this point, the movable support members 1705 are pushed upward by the springs 1611 (see FIGS. 15A to 15C) keeping the complex 1703 above the heat plate 1704 and the temperature increase of the complex 1703 at a slow rate.
In FIGS. 16BX and 16BY, a chamber 1709 is lowered until an elastic sheet 1706 attached to the chamber 1709 comes into contact with the complex 1703.
In FIGS. 16CX and 16CY, the chamber 1709 is further lowered until an O-ring 1719 attached to the chamber 1709 fits in an elastic sheet suction groove 1712 provided in the heat plate 1704. At the same time the chamber 1709 is lowered, a valve 1716 is opened to deaerate the interiors of a pipe 1714 and the elastic sheet suction groove 1712 by a vacuum pump 1718. In this way, the elastic sheet 1706 is fit over the heat plate 1704 by sucking and the complex 1703 is held inside an airtight space 1720 surrounded by the elastic sheet 1706 and the heat plate 1704. However, the complex 1703 is still not in contact with the heat plate 1704 and the temperature of the heat plate 1703 rises at a slow rate.
In FIGS. 16DX and 16DY, a valve 1717 is also opened to deaerate the interiors of a pipe 1715, deaeration holes 1713, and the airtight space 1720 created in FIGS. 16CX and 16CY by the vacuum pump 1718. As a result, the elastic sheet 1706 is tightly fit over the complex 1703 and accordingly thin metal wires 1702 are fixed to the top face of a photovoltaic member 1701. The deaeration also causes atmospheric pressure to compress the springs that have been pushing the movable support members 1705 upward, bringing the complex 1703 into contact with the heat plate 1704. The temperature of the complex 1703 is thus increased rapidly to the temperature level of the heat plate 1704 and the bonding phenomenon taking place between the thin metal wires 1702 and the photovoltaic member 1701 is advanced.
The state shown in FIGS. 16DX and 16DY is maintained for a given period of time until the thin metal wires 1702 are thoroughly bonded to the top face of the photovoltaic member 1701. Then the chamber 1709 is lifted to stretch the springs 1611 and move the complex 1703 away from the heat plate 1704. This lowers the temperature of the complex 1703. Thermocompression bonding of the thin metal wires 1702 to the photovoltaic member 1701 is thus completed.
According to this method, the heat plate 1704 is kept heated to start heating the photovoltaic member 1701 and the thin metal wires 1702 by putting the complex 1703 on the heat plate 1704 and to stop the heating by moving the complex 1703 away from the heat plate 1704. The photovoltaic member 1701 thus can be heated and cooled in a short period of time and therefore the method is suitable for mass production. This method is also superior for the ease with which an automatic hand, for example, one with claws, puts the photovoltaic member 1701 on the heat plate 1704 and moves the photovoltaic member 1701 away from the heat plate 1704 since the movable support members 1705 keep the complex 1703 above the heat plate 1704 on these occasions.
However, the conventional method described above is not always successful in bonding the thin metal wires 1702 by thermocompression to the photovoltaic member 1701 and sometimes the thin metal wires 1702 are bonded in a bent state to the top face of the photovoltaic member 1701 or are only partially bonded.
In the conventional method, at the stage shown in FIGS. 16CX and 16CY, the thin metal wires 1702 are not fixed yet except the ends fixed to the photovoltaic member 1701. Although the thin metal wires 1702 are taut owing to a tensile force applied in advance, the elasticity of the thin metal wires 1702 allows the wires to quiver like a string. When the airtight space 1720 is deaerated over a period from FIGS. 16CX and 16CY to FIGS. 16DX and 16DY, a change in gas pressure or a gas flow in the airtight space 1720 causes the thin metal wires 1702 to quiver. The elastic sheet 1706 presses the thin metal wires 1702 that are still quivering against the top face of the photovoltaic member 1701. Accordingly, the thin metal wires 1702 are sometimes fixed in a bent state to the top face of the photovoltaic member 1701.
If the elastic sheet 1706 is sagged when fit tightly over the complex 1703 and the heat plate 1704 by deaerating the airtight space 1720 over a period from FIGS. 16CX and 16CY to FIGS. 16DX and 16DY, a wrinkle 1721 may appear in the elastic sheet 1706 as shown in an enlarged view in FIG. 16DX. In this case, the force by which the thin metal wires 1702 are pressed against the photovoltaic member 1701 is weaker under the wrinkle 1721 and therefore the thin metal wires 1702 are insufficiently bonded to the photovoltaic member 1701 in this portion.
As a counter measure to this problem, the inventor of the present invention have thought of a method shown in FIGS. 17A to 17D. The method is capable of preventing the thin metal wires 1702 from quivering upon deaeration of the airtight space 1720 between the elastic sheet 1706 and the heat plate 1704 and accordingly preventing the elastic sheet from sagging. FIGS. 17A to 17D are sectional views corresponding to FIGS. 16AY to 16DY. FIG. 17A shows a state after completion of a first step in which a complex 1803 with thin metal wires 1802 fixed at each end onto a photovoltaic member 1801 is put on movable support members 1805 protruding from the principal surface of a heat plate 1804. FIGS. 17B to 17D show in stages how an elastic sheet 1806 is continuously pressed against the complex 1803. Throughout the stages shown in FIGS. 17B to 17D, the pressure applied to a surface of the elastic sheet 1806 that faces the heat plate 1804, namely, the top face of the elastic sheet in the drawings, is kept larger by means (not shown) than the pressure applied to a surface of the elastic sheet 1806 that is opposite to the heat plate 1804, namely, the bottom face of the elastic sheet in the drawings. The elastic sheet therefore bulges downward as shown in FIGS. 17A to 17D. At the stage shown in FIG. 17B, the elastic sheet stretched downward comes into contact with the central portion (an area A in the drawing) of the thin metal wires 1802 to apply a pressure to the area A. Therefore, the central portion of the thin metal wires 1802 is the first to be fixed to the photovoltaic member 1801. The thin metal wires 1802 are thus ironed out and fixed in a continuous manner from the central portion (area A in FIG. 17B) toward the ends over FIGS. 17B to 17D. At this point, the air between the elastic sheet 1806 and the heat plate 1804 is pushed rightward and leftward of FIGS. 17A to 17D.
This method does not need to deaerate the space between the elastic sheet 1806 and the heat plate 1804, and therefore the thin metal wires 1802 are prevented from quivering unlike the conventional method illustrated in FIGS. 16AX to 16DX and FIGS. 16AY to 16DY. Even if deaeration holes are provided in the heat plate 1804 as in FIGS. 16AX to 16DX and FIGS. 16AY to 16DY to deaerate the air between the elastic sheet 1806 and the heat plate 1804 after the step of FIG. 17D, the thin metal wires 1802 have already been pressed against the photovoltaic member 1801 by the elastic sheet 1806 by that point and are prevented from quivering. The method is thus capable of avoiding a situation in which the thin metal wires 1802 are fixed in a bent state onto the photovoltaic member 1801.
A tensile force is kept applied to the elastic sheet 1806 in FIGS. 17A to 17D since the pressure applied to the top face of the elastic sheet 1806 is always larger than the pressure applied to the bottom face. Under the constant application of tensile force, the elastic sheet 1806 is pressed against the complex 1803 and the heat plate 1804 continuously and radially from the central portion (area A in FIG. 17B). The elastic sheet 1806 is thus prevented from sagging and forming wrinkles unlike the related art, and accordingly the thin metal wires 1802 can be bonded to the photovoltaic member 1801 satisfactorily.
However, the method illustrated in FIGS. 17A to 17D has been found to allow thin metal wires to bond in a bent state in the case where a photovoltaic member has a thin substrate of insufficient rigidity from the following reason:
Since the complex 1803 is depressed by the elastic sheet 1806 bulging downward, the central portion of the complex 1803 is deformed to have a concave shape and to sink the movable support members 1805 in the central portion of the heat plate 1804 first. The concave deformation of the central portion eases the tensile force applied to the thin metal wires 1802, which are fixed to the surface of the complex 1803. With their tautness eased, the thin metal wires 1802 could be fixed in a bent state onto the photovoltaic member 1801 by the elastic sheet 1806.
If the thin metal wires 1702 are bonded in a bent state to the top face of the photovoltaic member 1701, Joule loss increases as the current collection path is lengthened. Furthermore, if the thin metal wires 1702 are bent and bonded to a light incidence face of the photovoltaic member 1701, shadows of the thin metal wires 1702 projected onto the light incidence face take up a large area and reduce the amount of electricity generated by the photovoltaic device.
In addition, when the thin metal wires 1702 are only partially bonded to the photovoltaic member 1701, the electric resistance is raised in a portion where the bonding is insufficient causing the photovoltaic device to generate electricity in a reduced amount.
Therefore, the second improvement that is desired to be made on the related art is to avoid bending of the thin metal wires 1702 and local bonding failure so that an increase in Joule loss and lowering of electricity generated by the photovoltaic device are prevented.